Portable hand-held metal detectors for searching and finding metal objects lying on or near the surface of the ground or buried just beneath the surface of the ground are becoming increasingly popular. Such detectors are moved or swept over the surface of the ground in front of the operator; and when the detector passes over a metal object, the disturbance of a magnetic field is used to cause the detector to produce a visual or audible indication of the presence of the metal.
A primary difficulty which exists in the use of metal detectors is that of distinguishing between metal objects which are valuable, such as coins, jewelry and the like, and metal objects having little value, such as aluminum foil, pull tabs, nails and other ferromagnetic materials. These latter objects can be classified as "trash" as contrasted to the desired or valuable objects which the detector is used to find.
Metal detectors which attempt to distinguish between valuable metal objects and trash have been developed. Since the reactive signal components of the signals detected by the receive coil of the popular transmit/receive metal detectors are 180.degree. out of phase between ferrous and non-ferrous metals, simple slicing circuits have been developed to produce output signals only for non-ferrous objects which are outside a quiescent condition of operation. In theory, such a simple system appears to offer the desired solution. In actual practice, however, the ground effects caused by ferromagnetic minerals in the soil, in the form of ferric oxide and other ferrous materials, create substantial difficulties in setting a proper slicing level because of the varying magnitude of such background of "earth effects" on the operation of the detectors.
To eliminate the "earth effects" or undesirable effects due to ferromagnetic minerals, systems have been developed which are insensitive to ground effects. This has been accomplished, however, at the expense of eliminating the ability of the metal detector to discriminate between ferrous and non-ferrous objects. This result occurs since ground effects produce primarily reactive signal components and produce very little resistive or eddy current signal components in the signal picked up by the receive coil. Metal targets, on the other hand, produce both resistive and reactive voltage components. The resistive signal components, however, produced in the receive coil by metal objects are the same irrespective of whether the objects are ferrous or non-ferrous. It is only the reactive signal components of the targets which provide a capability of distinguishing between desired (non-ferrous) and undesired (ferrous and non-ferrous) metal targets. Thus, if the ground effects are eliminated completely in prior art machines, the desired ability to distinguish between different types of metal targets is lost.
Some prior art metal detectors use a system of inverse discrimination to make the detector relatively insensitive to mineralization changes of the ground. Thus, detectors of this type can be set to be insensitive to undesired non-ferrous materials such as aluminum foil and pull tabs, and changes in the soil mineralization will not change this setting. This, however, is not ground effect rejection.
Other prior art metal detectors using discriminators plus AC coupling attempt to eliminate ground effects but are still subject to false readings if the detector is held on a spot and then pulled away. In addition, such detectors have a very shallow penetration into the ground which renders them of little value.
Another approach in the prior art has been to place two types of instruments in the same housing, that is, a very low frequency ground reject system and a separate discriminator/detector system, and then switch back and forth between the two to provide the operator with a composite set of signals from which he supposedly can determine whether the target found by the detector is a valid "good" target as opposed to an invalid or "bad" target. The ground reject circuit will detect relatively deep metal targets, but it has no discrimination capabilities. The discriminator circuit has discrimination capabilities but with very shallow depth. The result is that the composite instrument is no better than the discriminator circuit portion alone.
Accordingly, it is desirable to provide a metal detector which rejects the effects of ferromagnetic minerals in the soil being searched and which, at the same time, rejects ferrous metal targets along with junk (non-ferrous) metal targets to produce signals only for valuable non-ferrous metal targets such as coins, jewelry and the like.